Painted decorative finishes have long been available for various products in industries such as the automotive, appliance, consumer electronics, and household application industries. However, variations in paint deposition methods have resulted in the aesthetics of the final product being undesirable.
It is also known in the art that plastic work pieces may be masked prior to being painted with spraying mechanisms in order to provide a multiple painted finish, i.e., two-tone, appearance of the work piece. To provide a two-tone appearance on large work pieces like fascias or bumpers of automobiles, it is known that the work piece may be masked by manually applying a tape thereto prior to painting. Once the paint has cured, the tape may be peeled off revealing the two-tone appearance of the work piece. An issue with this masking technique is that it is very labor intensive and the consistency of the process can be poor since the tape is aligned and applied by hand.
To provide a two-tone appearance on smaller or more intricate work pieces such as grilles and wheel covers, it is known to cover portions of the work piece with rigid metal masks. Such masks provide for certain advantages over taping methods. Notably, they can be more quickly attached to the work piece and tend to be more consistently aligned since automated fixtures can be utilized to attach and remove the mask. The downside to using such masks, however, is that the transfer efficiency, i.e., the percentage of paint that is sprayed and which successfully makes it onto the work piece, can be quite low—sometimes less than 20%.
In traditional non-masked spray painting operations (that don't involve creating a two-tone appearance), it is known to utilize electrostatic spray painting techniques to improve the transfer efficiency to 55% or higher on work pieces. In such electrostatic spray painting operations, the work piece is generally made of a conductive material. The painting operation includes the steps of grounding the work piece, electrostatically charging a paint to render the paint conductive for statically attracting the conductive paint toward the grounded work piece, and spraying the work piece with the conductive paint. To allow plastic work pieces to be painted in such electrostatic spray painting operations, it is known to apply a conductive coating on the work piece prior to painting (e.g., as disclosed in U.S. Pat. No. 6,455,110 to Fortuyn et al.).
Metal masks are generally not utilized in electrostatic spray painting operations to provide a two-tone appearance of work pieces because the metal masks are conductive, which can cause a large amount of paint to be attracted to and applied to the mask instead of the work piece.
As such, there remains a need for improved methods of painting plastic work pieces to yield a two-tone appearance that are efficient and provide for high transfer efficiencies.